Graphics-intensive applications for computers such as personal computers (PC's) are becoming increasingly more popular. Such applications include high-end computer-aided drafting (CAD) applications, games, real-time video applications, as well as other applications. As these applications become more complex, they require the computers on which they are run to render and execute graphics much more quickly. Furthermore, as the typical resolution of computer screens have increased from 640.times.480 pixels (horizontal.times.vertical) to 800.times.600, 1024.times.768 and beyond, and increased color information per pixel from two bits to 24 bits to 32 bits and beyond, the processing demand placed on the computers for fast graphics execution has also grown.
The typical computer relies on a graphics subsystem (also known as a graphics card, video card, graphic accelerator card, or a display adapter, among other terms) to assist it in the display of graphics on a display device. A graphics card generally includes a specialized processor or processors that are tailor-made for graphics rendering, as well as an amount of memory, ranging from one, two, four, eight, sixteen megabytes and up, so that a complete screen of graphics information, known as a frame, can be stored by the graphics card. Thus, this memory is generally known as a frame buffer of the graphics card. Graphics "cards", may also be integrated within a single chip on a motherboard of a computer. A graphics card, and potentially other components, make up the graphics subsystem of a computer. The entirety of the graphics subsystem may also be integrated into a single chip on the motherboard of a computer. The graphic subsystem and other computer components from the motherboard may alternatively be embedded in a semiconductor chip.
Initially, the memory of a graphics card was standard-issue dynamic random-access memory (DRAM), of a sort also used by computer processors to hold more general information. Thus, as improvements in memory to increase their speed became available, such as the introduction of synchronous dynamic random-access memory (SDRAM), they usually have been utilized within graphics cards, too. Ultimately, however, the specialized needs of graphics rendering required their own type of memory, such as synchronous graphics random-access memory (SGRAM), which is analogous to SDRAM, but includes enhanced graphics features for use with graphics cards. The need for faster memory within graphics cards has not, however, abated.
Thus, graphics cards manufacturers have looked to new technologies, such as Rambus DRAM's (also known as Direct RDRAM's), available from Rambus, Inc. of Mountain View, Calif., to increase graphics subsystem performance. Rambus DRAM use within graphics cards, however, has been limited because it is based on a closed standard governed by Rambus, Inc., such that use of Rambus DRAM requires the payment of royalties to Rambus, Inc. Therefore, manufacturers have looked to other technologies that are based on open standards.
One such type of memory is the Double Data Rate (DDR) DRAM. The DDR DRAM achieves increased performance by providing for two data accesses within a single clock cycle --hence its name --by enabling the memory to read data on both the rising and falling edges of each clock cycle. The concept of DDR memories has been extended to SDRAM's and SGRAM's in particular, resulting in DDR SDRAM and DDR SGRAM. Such memory has witnessed increased interest on the part of graphics card designers as a manner by which increased graphics performance can be realized.
A disadvantage to DDR SDRAM/SGRAM's found in the prior art, however, is that full-page burst is not generally provided for. (In general, a burst operation for a memory is defined as an operation retrieving a given number of data stored at sequential locations within the memory (e.g., a full-page of memory), which ultimately allows the data to be retrieved in a faster manner --hence the term "burst.") This is because of a limitation of the prefetch nature of DDR devices. That is, in a DDR device, for a given clock cycle, two words of data, each of n-bits length, are retrieved, such that both must be from the same location within the memory as addressed by the logical circuitry before the next location can be moved to. This is acceptable for a full-page increment burst starting with an even start word address, because the second data word retrieved is still within the same location within the memory as the first data word. However, a full-page increment burst starting with an odd start word address does not work, because the second data word retrieved will necessarily not lie within the same location as the first data word as addressed by the logical circuitry, violating the limitation of the prefetch nature of DDR devices.
There is a need, therefore, for a DDR SDRAM/SGRAM that has burst capability. For these and other reasons, there is a need for the present invention.